Unlike  nuclear  fallout.  the  dispersal  of  chemical contamination  is  normally  confined  to  a  far  smaller area,  with  much  of  the  effects  of  the  chemical depending   on   the   local   weather   conditions. Considering   the   limited   area   effected   by   chemical weapons,   any   plot   of   chemical   contamination   is normally done only on the smaller scale charts, such as 1:50,000. In  amphibious  battle  situations  supported  by  U.S. Naval ships and Marine Corps forces, you must prepare for the possibility that the opposing force will employ chemical weapons either against the ships or against the ground forces supported by your ship. The attack may come  from  bombs,  rockets.  or  aerial  spray.  Accurate calculations for the area contaminated by the chemical agent  are  only  possible  after  the  agent  has  been identified.   These   calculations   are   preformed   by forecasters  or  NBC  evaluators  following  procedures detailed in ATP-45. Evaluated information is passed as a  message  following  the  NBC  3  CHEM  format.  As  a Aerographer,  you  should  be  able  to  interpret  a  NAV NBC  3  CHEM  or  an  NBC  3  CHEM  report—the Chemical Downwind Message, and to plot the possible contamination area reported in the message. Later, as you study to become a forecaster, you will learn  about  the  format  of  a  Chemical  Downwind Forecast  (CDF)—a  product  of  basic  forecast meteorological  parameters  used  to  evaluate  how  far downwind   a   particular   chemical   agent   remains hazardous.  CDFs  are  normally  used  by  non- meteorological  personnel  at  NBC  centers  to  evaluate the duration and extent of chemical contamination for a specific chemical agent. CHEMICAL  WARFARE  TERMS Chemical  agents  may  be  spread  by  many  means, such  as  bombs,  mortar  shells,  artillery  shells,  rockets, missiles.  mines.  generator  fog,  or  aircraft  spray.  The area where the chemical agent is released is called the attack area. The attack area includes a minimum radius of  1/2  nautical  milc  (at  sea)  or  1  kilometer  (ashore) around  the  release  site.  This  area  is  considered  to  be immediately  contaminated  because  of  the  explosives distributing the agent or by the initial chemical spray. Chemical  agents  are  spread  outward  from  the  attack area by diffusion through the air and by mixing caused by  the  winds.  The  area  contaminated  by  airborne chemical  agents  outside  the  attack  area  is  called  the hazard  area.  In situations involving calm or light winds (5  knots  or  less),  the  contamination  is  considered  to spread outward from the attack area in all directions. When the winds are greater than 5 knots, describing the hazard area is not as simple. Generally, the hazard area will form a triangular pattern. The contamination on  the  upwind  side  of  the  attack  area  is  prevented because  the  contamination  is  carried  with  the  wind toward  the  downwind  side  of  the  attack  area.  The information  needed  to  describe  the  triangular  pattern and the orientation of the hazard triangle is the chemical downwind  direction,  the  downwind  hazard  distance, and the radial angles. The  direction  the  contamination  is  carried  by  the wind  is  called  the chemical downwind direction  (CDD). The  CDD  is  180  degrees  opposite  the  surface-wind direction. The  distance  the  contamination  is  carried  by  the wind,  outside  the  attack  area,  in  high  enough concentration   to   cause   casualties,   is   called   the downwind hazard distance (DHD), as shown in figure 5-9. The DHD depends on the type of agent, the means of   delivery,   the   terrain,   and   the   effects   of   the meteorological  elements  on  the  agent.  The  DHD  is  a minimum of 5 nautical miles (10 kilometers) and may extend up to 27 nautical miles (SO kilometers) from the center of the attack area. Radials extend tangent from the edge of the attack area 30 degrees either side of the CDD ashore. At sea, 35° radial angles are used when the winds are between 5 and 10 knots, but only 20° radial angles are used when the winds are 10 knots or greater. The angles used to draw the radials account for variations in wind direction and horizontal diffusion of the chemical agent. These angles are referred to as half-sector angles (HSA). The  chemical  down  windspeed  (CDS)  is  considered the  same  as  the  prevailing  surface  wind  speed. However,  calculations  involving  the  time  the  leading edge of a chemical cloud arrives at a specific downwind location use a maximum speed of the CDS multiplied by 1.5 in order to account for variability in wind speed or gusty conditions. One  basic  characteristic  of  a  chemical  agent  that must be considered is the persistency of the agent—the tendency of the agent to vaporize to form a hazardous gas.  Non-persistent  (NP)  agents  are  generally hazardous  gases  that  remain  in  the  target  area  for minutes or, in exceptional cases. for several hours after the   attack.   Persistent   (P)   agents   are   generally hazardous liquids which usually emit a hazardous vapor that remains in the target area for hours or days or, in exceptional cases, weeks after the attack. 5-10